If each gym was assigned a random type like they do in the real games, gyms could have different visual features that would make each gym feel more distinct like blazing furnaces on top of fire gyms or having grass gyms covered in vines or steel type looking more industrial, etc. They could do things like increase the power of pokemon you put in the gym that match the type of gym and give you double coins for matching the type. Would make gyms much more interesting because you wouldn't just throw your highest defense mon in every gym and makes pokemon you wouldn't normally use suddenly good for defending gyms. I can't think of a single downside to this.
>Wouldn't that just make it really easy to attack gyms with all of the same type?
Yeah probably, but most people just use whatever the game suggests for them to fight with anyways.